Problem: Simplify the following expression: $k = \dfrac{1}{9a} + \dfrac{1}{8a}$
Solution: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9a$ and $8a$ $\lcm(9a, 8a) = 72a$ $ k = \dfrac{8}{8} \cdot \dfrac{1}{9a} + \dfrac{9}{9} \cdot \dfrac{1}{8a} $ $k = \dfrac{8}{72a} + \dfrac{9}{72a}$ $k = \dfrac{8 +9}{72a}$ $k = \dfrac{17}{72a}$